Thermoplastic polymers, e.g., polyester and polypropylene, are known to be able to be melt extruded into monofilament and strand of denier greater than 15 denier per filament (dpf), especially greater than 30 dpf.
Convenient references which relate to fibers and filaments, including those of man-made thermoplastics, and incorporated herein by reference, are, for example:
(a) Encyclopedia of Polymer Science and Technology, Interscience, New York, Vol. 6 (1967) pp. 505-555 and Vol. 9 (1968) pp. 403-440. PA0 (b) Man-Made Fiber and Textile Dictionary, published by Celanese Corporation: PA0 (c) Fundamentals of Fibre Formation--The Science of Fibre Spinning and Drawing, by Andrzij Ziabicki, published by John Wiley & Sons, London/New York, 1976: PA0 (d) Man-Made Fibres, by R. W. Moncrieff, published by John Wiley & Sons. London/New York, 1975: PA0 (e) Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 16 for "Olefin Fibers" published by John Wiley & Sons, New York, 1981, 3rd Edition.
In conformity with commonly accepted vernacular or jargon of the fiber and filament industry, the following definitions apply to the terms used in this disclosure:
A "monofilament" (a.k.a. monofil) refers to an extruded individual strand of denier greater than 15, usually greater than 30;
A "fine denier fiber or filament" refers to an extruded strand of denier less than about 15;
A "multi-filament" (a.k.a. multifil) refers to simultaneously extruded fine denier filaments formed as a bundle of fibers, generally containing at least 3, preferably at least about 15-100 fibers and can be several hundred or several thousand;
"Staple fibers" refers to fine denier strands which have been formed at, or cut to, staple lengths of generally about 1 to about 8 inches;
An "extruded strand" refers to an extrudate formed by passing polymer through a forming-orifice, such as a die.
Practitioners are aware that it is easier to make a coarse monofilament yarn of 15 denier than to make a multi-filament yarn of 15 denier. It is also recognized that the mechanical and thermal conditions experienced by a bundle of filaments, whether in spinning staple fibers or in multi-filament yarns, are very different to those in spinning monofilaments. The fact that a given man-made polymer can be extruded as a monofilament, does not necessarily herald its use in fine denier or multi-filament extrudates.
Linear low density polyethylene (LLDPE) has been shown to be melt spun into fine diameter fiber, i.e., fiber denier less than about 15 dpf, e.g. in U.S. Pat. No. 4,830,907, incorporated herein by reference. Difficulties arise, however, when one tries to melt spin LLDPE to a denier size less than about 6 dpf for a specified length of time, while minimizing filament breaks. Spinning LLDPE to a fiber size of less than about 6 dpf, preferably to about 3-4 dpf is especially desirable in personal care types of products, such as baby diaper coverstock and feminine hygiene products, to ensure uniform coverage at low basis weights.
Melt spinning LLDPE into fine denier fiber, e.g., 15 dpf, and subsequently mechanically drawing is not acceptable for obtaining fiber having a denier less than about 6 dpf, especially when that fiber is utilized in a thermal bonding application. Not only does the secondary drawing step create an additional process limitation, but also causes high shrinkage of that fiber when the fiber is later thermally bonded into fabric, creating poorly bonded fabric.
When the fiber is made via a spunbond process, a secondary drawing step is not an option and the fiber must be melt drawn without fiber breakage to its final diameter. In spunbond applications such as baby diaper coverstock, the desired final diameter is usually about 3-4 dpf. The small diameter ensures uniform coverage of the fabric and adequate bonded fabric strength from the fibers which are thermally bonded.
Many process limitations or choices exist in a staple fiber manufacturing line or in a spunbond fabric manufacturing line, and many of these have been optimized for melt spinning a particular type of polymer for maximum effect. These process steps include extruder screw design, polymer melt spinning temperature, quench (i.e., cooling) temperature, polymer throughput, spinneret design (e.g., die hole size, entrance angle, and hole concentration), and of course, polymer design.
One process limitation, the spin pack filtration media, has traditionally been used to remove gross contamination and prevent die plugging in melt spinning various polymers. The spinnerets used in the melt extrusion usually have very small diameter holes (on the order of approximately 600 micrometers diameter) and polymer contamination can create plugging of the spinneret and subsequent equipment shutdown.
Historically, thermoplastic polymers have utilized various kinds of filtration media, usually commensurate with the degree of contamination. Typical melt spinning filtration consists of multilayered screen packs, sand, bauxite type filtration and sintered metal filters. Modern polymers such as polypropylene or polyethylene typically use a three layer screen pack (e.g., 325/60/20 mesh screens sandwiched together) immediately prior to extrusion through the spinneret die to remove incidental gross contamination, since modern polymers are generally free of fine particulate contamination. The function of the filtration media has been to remove this incidental gross contamination (e.g., dirt or insoluble particles). Modern polymers, however, are usually uncontaminated with respect to fine particles, precluding the need for filtration finer than the three layer screen packs described above.
Melt spinning linear polyethylene, especially LLDPE, using little or no filtration (e.g., single layer screen packs) is difficult, especially when trying to spin into finer denier fiber (i.e., fiber having a denier of less than about 6 dpf) with minimal fiber breaks. The choice of filtration media immediately prior to extrusion through a spinneret die has now been discovered to affect the ability of linear ethylene polymers to be melt drawn into finer diameter fiber.